1. Field of the Invention
The present invention relates to a method of judging a propriety of a printing position of a printing head, which is designed to reciprocate and print in both directions of the reciprocation on a recording sheet, and a printing apparatus for carrying out this method. In particular, the present invention relates to a method of judging a propriety of a relative positional relation between the printing operation in the forward direction and the printing operation in the backward direction performed by the printing head, and a printing apparatus for carrying out this method.
2. Description of the Related Art
For example, in a printing apparatus having an ink-jet type or wire-dot type printing head, during a printing operation, as a recording sheet is sent or fed intermittently to a prescribed direction, the printing head moves back and forth in a direction perpendicular to the direction in which the recording sheet is sent. In this way, the printing head performs a desired printing operation on the entire printing region on the recording sheet.
In order to improve the printing operation efficiency, the printing head is designed to print in both of the directions of the reciprocation (i.e. forward and backward directions). In this case, the relative positional relation between the printing operation in the forward direction and the printing operation in the backward direction by the printing head needs to be set properly. When the relative positional relation is not properly set, a discrepancy arises between the printing operation in the forward direction and the printing operation in the backward direction. In this case, for example, when a vertical ruled line is printed across several lines or horizontal rows, the vertical ruled line does not turn out to be a straight line. In an extreme case, the vertical ruled line appears as several vertical ruled lines which are displaced or shifted from each other with respect to the direction of the motion of the printing head.
However, it is relatively difficult to set properly the relative positional relation between the printing operation in the forward direction and the printing operation in the backward direction. This difficulty is primarily caused by the fact that the printing head keeps on moving during the printing operation. Since a gap must always exist between the printing head and the recording sheet, there is always a time difference between the timing at which the printing head starts printing and the timing at which the printing is finished on the recording sheet. This time difference also contributes to this difficulty. In addition, the gap between the printing head and the recording sheet varies due to the dimension errors that parts containing the printing head have, an irregularity of the installation position of the printing head, the thickness of the recording sheet being used, or the like. This gap also contributes to this difficulty.
For example, the ink-jet type printing head will be explained more specifically. When the printing head injects ink, the ink reaches the recording sheet after some time interval or delay. Since the printing head is moving during the printing operation, the actual printing position on the recording sheet is shifted forward in the direction of the motion of the printing head with respect to the position at which the printing head has injected the ink. This shift amount is determined by the velocity of the moving printing head, the size of the gap between the printing head and the recording sheet, and the injection speed of the ink that travels this gap. Of these determining factors, the size of the gap between the printing head and the recording sheet varies depending on the errors contained in the parts or components, the irregularity in the installment position of the printing head, or the thickness of the recording sheet being used. This variation of the size of the gap also causes the shift amount to vary.
In addition, as has been explained above, this shift is generated on the forward side in the direction of the motion of the printing head with respect to the position at which the printing head has actually injected the ink. Therefore, it is to be noted that the side toward which the printing position shifts when the printing head moves in the forward direction is opposite to the side toward which the printing position shifts when the printing head moves in the backward direction. As a result, for example, if the shift amount varies by a specific amount due to the variation of the size of the gap between the printing head and the recording sheet, the magnitude of this variation appears as twice that of the shift amount variation, when the shift amount is viewed between the printing operation in the forward direction and the printing operation in the backward direction.
Under these circumstances, before a printing apparatus having a printing head that moves backward and forward is shipped, the propriety of the relative positional relation between the printing operation in the forward direction and the printing operation in the backward direction is judged. If the relative positional relation has been judged to be improper, the relative positional relation is adjusted so as to become proper. This adjustment is performed by changing the amount of delay between the timing at which the motion of the printing head starts and the timing at which the printing head starts printing.
Moreover, at the above-mentioned pre-shipment stage, even if the relative positional relation between the printing operation in the forward direction and the printing operation in the backward direction is properly adjusted, this relative positional relation can start to become improper while the printing apparatus is being used. For example, at the pre-shipment stage, the relative positional relation is adjusted and set properly using a recording sheet having a standard thickness. However, in the case in which the user uses a recording sheet of a non-standard thickness, or though it occurs rarely, in the case in which the installation position of the printing head has moved due to some factors, or the like, this relative positional relation needs to be re-adjusted.
In adjusting the relative positional relation at each of these stages, the actual printing is performed on a recording sheet. Based on this printing result, an adjustment value that will eliminate the discrepancy between the printing operation in the forward direction and the printing operation in the backward direction is determined. Based on this adjustment value, the delay amount between the timing at which the printing head starts moving and the timing at which the printing head starts printing is changed. This method is adopted in adjusting the relative positional relation. In order to determine the adjustment value that will eliminate the discrepancy between the printing operation in the forward direction and the printing operation in the backward direction based on this printing result as described above, the propriety of the relative positional relation between the printing operation in the forward direction and the printing operation in the backward direction needs to be judged by a visual observation. In order to render this judgment, the following method is adopted.
Namely, as shown in FIG. 21, by carrying out the actual printing, for example, three horizontal rows (i.e. three lines) of vertical ruled line patterns are formed on a recording sheet. More specifically, in the first row (i.e. the top row), multiple vertical ruled lines 1 are printed along a direction indicated by an arrow 2. Next, after the row is changed as shown by an arrow 3 (i.e., after a line feed is performed), the printing direction is changed as shown by an arrow 4. The same number of multiple vertical ruled lines 5 are then printed in the second row (i.e. the middle row). After the row is changed again as shown by an arrow 6 (i.e., after the line feed is performed again), as shown by the arrow 2, the same number of multiple vertical ruled lines 7 are printed in the third row (i.e. the bottom row) in the same printing direction as the first row.
In these ruled line patterns, if the vertical ruled lines 1 in the first row, the vertical ruled lines 5 in the second row, and the vertical ruled lines 7 in the third row are all collinear, the relative positional relation between the printing operation in the forward direction and the printing operation in the backward direction is judged to be proper. In FIG. 21, since the vertical ruled lines 1, 5 and 7 are not collinear, the relative positional relation between the printing operation in the forward direction and the printing operation in the backward direction is judged to be improper. Therefore, an adjustment value is selected so that these vertical ruled lines 1, 5 and 7 will become collinear. In accordance with this adjustment value, for example, the delay amount between the timing at which the printing head starts moving and the timing at which the printing head starts printing is changed and corrected.
In judging the propriety of the above-described printing positions, judgment patterns that have actually printed are visually observed by a human.
However, it is extremely difficult to judge the propriety of the printing positions by such a visual observation using the above-described ruled line patterns shown in FIG. 21. In FIG. 21, the state in which the relative positional relation between the printing operation in the forward direction and the printing operation in the backward direction is improper is exaggerated for the sake of explanation. Therefore, the sizes of the displacements among the vertical ruled lines 1, 5 and 7 are shown quite large. However, in reality, in order to judge the propriety of the relative positional relation, it must be judged whether the vertical ruled lines 1, 5 and 7 are collinear even if, for example, these vertical ruled lines are displaced by the thickness or width of the vertical ruled line. Hence, it is extremely difficult for the visual observer to judge confidently the propriety with a high degree of accuracy.
In addition, the following problems also arise in judging the propriety of the printing positions using the above-described ruled line patterns. These problems will be explained with reference to FIGS. 22, 23 and 24, respectively. In FIGS. 22, 23 and 24, the same elements as those in FIG. 21 carry the same reference numerals, and the explanations thereof are omitted.
FIG. 22 illustrates a printing example of ruled line patterns which are generated when the printing head is inclined in the direction of the motion of the printing head. Under this circumstance, the ruled lines 1, 5 and 7 cannot become collinear. Hence, it is impossible to even select an adjustment value that will make the ruled lines 1, 5 and 7 collinear.
FIG. 23 illustrates a printing example of ruled line patterns when the head surface of the printing head is not parallel to the recording surface. In this case, the ruled lines 1, 5 and 7 can be connected to form a single line. An adjustment value for creating this state exists. However, even though the ruled lines 1, 5 and 7 are connected forming a single line, the line is not a straight line. Therefore, it is extremely difficult to judge this state.
FIG. 24 illustrates a printing example of ruled line patterns in the case in which the velocity of the motion of the printing head has changed. Under this circumstance, some of the multiple vertical ruled lines 1 become collinear with some of the corresponding multiple vertical ruled lines 5 and 7, and the remainder of the multiple vertical ruled lines 1 do not become collinear with the corresponding multiple vertical ruled lines 5 and 7. Therefore, it is extremely difficult to judge the propriety of the relative positional relation.